Britain’s Prince William and Kate Middleton have announced that they’re expecting a second child. Just as during her first pregnancy, the Duchess of Cambridge is suffering from hyperemesis gravidarum, a rare form of morning sickness. Learn about this uncommon condition, and why severe nausea during pregnancy could be a sign of twins...

Could the world’s most famous royal couple be expecting twins? Or even triplets?

On Sept. 8, Prince William announced that he and Kate Middleton, Britain’s Duke and Duchess of Cambridge, are expecting a second baby.

And once again, Kate is experiencing an extreme form of morning sickness called hyperemesis gravidarum (HG), William’s office told reporters.

During her first pregnancy a year and a half ago, Kate was admitted to London’s King Edward VII Hospital for treatment of the condition. This time, she’s under the care of doctors in Kensington Palace, the couple’s London home, a spokeswoman said.

Because the condition causes severe nausea and vomiting, it can lead to dehydration and nutritional deficiencies, according to Maria Ottavi, M.D., an OB-GYN at Saint John’s Health Center in Santa Monica, Calif.

Hyperemesis gravidarum often signifies that the mother-to-be is carrying more than one baby – although that turned out not to be the case when Prince George was born.

Here are expert answers to your most-asked questions about the duchess’ pregnancy and her medical condition.

What are the symptoms of hyperemesis gravidarum?
Women with HG experience “tremendous vomiting and nausea” during pregnancy, Dr. Ottavi says.

As a result, they “can’t keep down even fluids, much less solids, so they may need medication and hydration,” she says.

They may also experience a drop in blood pressure and a slightly faster heart rate, leading to feelings of lightheadedness.

That’s why Kate was hospitalized in 2013. In addition to receiving hydrating fluids, she probably had lab tests to rule out other conditions, including thyroid problems, Dr. Ottavi explains.

Symptoms start during the first few months of pregnancy and usually die down after the first trimester, or at most the fourth month.

“Most women will go on to have a normal pregnancy after that,” Dr. Ottavi says.

What causes hyperemesis gravidarum?It seems to be due to “a sudden surge in progesterone and a pregnancy hormone called beta HCG (human chorionic gonadotropin),” Dr. Ottavi says. (That’s the same hormone that some people take as part of an unproven diet plan.)

How many women experience hyperemesis gravidarum?While most pregnant women experience some morning sickness, HG is fairly rare, occurring in anywhere from 3 to 20 per 1,000 pregnancies each year, according to the National Institutes of Health.

About 60,000 cases are reported in U.S. hospitals every year, although many more women are treated for it at home or in doctor’s offices, according to the American Pregnancy Association.

Even if a woman with HG has “weird food cravings,” such as a desire to eat only mashed potatoes or watermelon, she should eat to keep her calories and hydration up, Dr. Bohn adds.

A doctor needs to make sure the woman doesn’t lose weight because of nausea during pregnancy, Dr. Ottavi says.

“If she loses 10 pounds, it can be quite dangerous,” she says.

Does hyperemesis gravidarum increase the likelihood that Kate will have twins?Higher levels of HCG can result from carrying more than one fetus, and women with HG are more likely to have twins, according to Dr. Ottavi.

Kate’s doctors will probably order an ultrasound, which will show if she’s expecting more than one baby – the older of whom, theoretically, will become fourth in line to the British throne.

Women with HG are also slightly more likely to give birth to a girl, according to a July 2012 study at James Paget Hospital in England that was published in the Journal of Obstetrics and Gynaecology.

Are You Ready for Your New Baby?Your belly is bulging and your due date is fast approaching. Your bag is packed, the baby’s room is done, and you have a name picked out. But are you ready for your new baby? Take this quiz and find out.

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